Implantable medical electrical stimulation and/or sensing leads are well known in the fields of cardiac stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation, and in other fields of electrical stimulation or monitoring of electrical signals or other physiologic parameters of the body. A pacemaker or cardioverter/defibrillator implantable pulse generator (IPG) or a cardiac monitor is typically coupled to the heart through one or more of such endocardial leads. The proximal end of such leads typically is formed with a connector which connects to a terminal of the IPG or monitor. The lead body typically comprises one or more insulated, conductive wire surrounded by an insulating outer sleeve. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. An endocardial cardiac lead having a single stimulation and/or sensing electrode at the distal lead end and a single conductive wire is referred to as a unipolar lead. An endocardial cardiac lead having two or more stimulation and/or sensing electrodes at the distal lead end and two or more conductive wires is referred to as a bipolar lead or a multi-polar lead, respectively.
In order to implant an endocardial lead within a heart chamber, a transvenous approach is utilized wherein the lead is inserted into and passed through a pathway comprising the subclavian, jugular, or cephalic vein and through the superior vena cava into the right atrium or ventricle. It is necessary to accurately position the sense and/or stimulation electrode surface against the endocardium or within the myocardium at the desired site in order to achieve reliable sensing of the cardiac electrogram and/or to apply stimulation that effectively paces or cardioverts the heart chamber. The desired heart sites include the right atrium, typically the right atrial appendage, the right ventricle, typically the ventricular apex, and the coronary sinus and great vein descending therefrom.
The heart beats approximately 100,000 times per day or over 30 million times a year, and each beat stresses at least the distal portion of the lead. Over the years of implantation, the lead conductors and insulation are subjected to cumulative mechanical stresses, as well as material reactions as described below, that can result in degradation of the insulation or fractures of the lead conductors with untoward effects on device performance and patient well being. The endocardial lead body is subjected to continuous stretching and flexing as the heart contracts and relaxes and is formed to be highly flexible and durable. It is necessary to temporarily stiffen the lead body while advancing it through the pathway to locate the distal electrode(s) at the desired site.
Early implantable, endocardial and epicardial, bipolar cardiac pacing leads employed separate coiled wire conductors in a side by side configuration in the lead body and incorporated a lumen for receiving a stiffening stylet inside the lumen of at least one of the conductor coils to facilitate implantation. The stiffening stylet was advanced through a proximal connector pin opening to stiffen the lead body during the transvenous introduction and location of the distal electrodes deeply inserted into the right ventricular apex and was then withdrawn. The relatively large diameter and stiff lead body provided column strength that was relied upon to maintain the distal electrodes embedded into the trabeculae of the right ventricular apex. Fibrous tissue growth about the distal lead body was also relied upon to hold the distal pace/sense electrodes in position. Similar atrial, J-shaped lead bodies were developed that relied upon the lead body stiffness and shape to lodge and maintain distal pace/sense electrodes lodged into the right atrial appendage.
Such relatively large and stiff lead bodies were disadvantageous in a number of respects. The large diameter body made it difficult to implant more than one lead through the venous system. The column strength of such relatively large and stiff lead bodies often was insufficient to maintain the pace/sense electrodes in the atrial appendage or ventricular apex, and physicians often resorted to leaving the stylets in place, resulting in fracture of the lead bodies. Once the lead bodies fibrosed in, they were difficult to retract from the heart if they needed to be replaced. Thus, efforts were undertaken to solve all of these problems.
Considerable effort has been expended over the years to develop passive and active fixation mechanisms that are incorporated into the distal end of the endocardial lead to fix the electrode at a desired site in a heart chamber during an acute postoperative phase wile fibrous tissue growth tends to envelop the lead body. Passive fixation mechanisms do not invade the myocardium but cooperate with cardiac tissue or structures to locate the electrode or electrodes in contact with the endocardium. The most successful passive fixation mechanism comprises a plurality of soft, pliant tines that bear against cardiac structure surfaces, e.g. the trabeculae in the right ventricle and the atrial appendage, to urge the distal tip electrode against the endocardium. Active fixation mechanisms are designed to penetrate the endocardial surface and lodge in the myocardium without perforating through the epicardium or into an adjoining chamber. The most widely used active fixation mechanism employs a sharpened helix, which typically also constitutes the distal tip electrode. A shroud or retraction mechanism is provided to shield the helix during the transvenous advancement into the desired heart chamber from which the helix can be advanced and rotated when the desired site is reached to effect a penetrating, screw-in fixation. In one manner or another, the helix is adapted to be rotated by some means from the proximal end of the lead outside the body in order to screw the helix into the myocardium and permanently fix the electrode at the desired atrial or ventricular site.
Lead body design has progressed significantly over the years in the effort to increase longevity and flexibility and to diminish lead body size, while maintaining pull out strength to enable retraction of the lead body from the heart and increasing the number of conductors to distal electrodes or sensors. Co-axial, bipolar, coil lead bodies, of the type shown in U.S. Pat. No. 3,788,329, incorporated herein by reference, are widely used, wherein the separate coiled wire conductors are wound in differing diameters separated from one another by tubular insulating sheaths and extend coaxially about a central lumen for receiving the stiffening stylet. More recently, each such coiled wire conductor of both unipolar and bipolar leads is formed of a plurality of multi-filar, parallel-wound, coiled wire conductors electrically connected in common in an electrically redundant fashion. Such redundant coiled wire conductors of bipolar and multi-polar lead bodies are coaxially arranged about the stiffening stylet receiving lumen and insulated from one another by coaxially arranged insulating sheaths separating each coiled wire conductor from the adjacent coiled wire conductor(s).
In the implantation of a cardiac device of the types listed above, and in the replacement of previously implanted cardiac leads, two or more transvenous cardiac leads are typically introduced through the venous system into the right chambers or coronary sinus of the heart. It has long been desired to minimize the diameter of the transvenous cardiac lead body to facilitate the introduction of several cardiac leads by the same transvenous approach. Moreover, a number of multi-polar, endocardial cardiac leads have been designed to accommodate more than two electrodes or to make electrical connection with other components, e.g., blood pressure sensors, temperature sensors, pH sensors, or the like, in the distal portion of the lead. The increased number of separate polarity coiled wire conductors is difficult to accommodate in the conventional coaxial coiled wire conductor winding arrangement employing tubular insulating sheaths to separate the coil wire conductors of differing diameters having a desired overall lead body outer diameter.
These needs have led to the development of separately insulated, coiled wire conductors that are parallel-wound with a common diameter and are separately coupled between a proximal connector element and to a distal electrode or terminal in the manner described in commonly assigned U.S. Pat. No. 5,007,435, for example. The coaxial construction technique may also be combined with the parallel-winding technique to multiply the total number of separate coiled wire conductors accommodated within a specified endocardial lead body outer diameter.
All of the above considerations as to the complexity of the leads, the number of leads implanted in a common path, and the advancement of coronary sinus leads deep in the coronary veins have led to efforts to at least not increase and optimally to decrease the overall diameter of the cardiac lead body without sacrificing reliability and usability. It has been proposed to diminish the lead body further by eliminating the lumen for receiving the stiffening stylet and by replacing the coiled wire conductor with highly conductive stranded filament wires or cables. In bipolar or multi-polar leads, each such cable extends through a separate lumen of the lead body to maintain electrical isolation. Without the ability to use the stiffening stylet, it is necessary to resort to use of another mechanism to pass the lead through the vessel paths identified above and to position and fix the distal electrode of the lead at the desired site in the heart chamber or vessel. Moreover, the decreased lead body diameter and increased lead body flexibility reduces the column strength of the lead body and necessitates use of an active or passive fixation mechanism. Commonly assigned U.S. Pat. No. 5,246,014, incorporated herein by reference, presents a number of alternative designs of such straight, stranded filament wires used in small diameter lead bodies having an active fixation, distal tip electrode. An introducer surrounding the lead body, rather than a stiffening stylet in a lead body lumen, and engaging the distal active fixation mechanism is used to introduce and fix he electrode at the desired site.
A large number of endocardial pacing and cardioversion/defibrillation leads have also been developed that are adapted to be advanced into the coronary sinus and coronary veins branching therefrom in order to locate the distal electrode(s) adjacent to the left ventricle or the left atrium. The distal ends of such coronary sinus leads are advanced through the superior vena cava, the right atrium, the valve of the coronary sinus, the coronary sinus, and into a coronary vein communicating with the coronary sinus, such as the great vein. Typically, coronary sinus leads that have been released for clinical use and bearing relatively small, ring-shaped pace/sense electrodes and/or larger, elongated, cardioversion/defibrillation electrodes have straight lead bodies and electrodes. Moreover, they do not employ any fixation mechanism and instead rely on the close confinement within these vessels and the column strength of the lead body extending back to the IPG or cardiac monitor to maintain each such electrode at a desired site. Such a cardioversion/defibrillation coronary sinus lead is disclosed in commonly assigned U.S. Pat. No. 5,174,288, incorporated herein by reference.
A number of considerations are important in the design of coronary sinus leads. It is desirable to avoid completely obstructing the coronary sinus and veins extending therefrom the vein, and therefore it is not desirable to encourage and rely upon tissue growth to stabilize the electrodes located therein. It is not desirable to damage or penetrate or perforate the vein wall which itself would contribute to tissue growth and obstruction. However, it is desirable to obtain and maintain a precise location of the electrode(s) so that they can be used to accurately sense electrical activity of the left atrium or ventricle and to apply localized electrical stimulation thereto.
These considerations have led to variations on coronary sinus lead body and electrode designs as disclosed in commonly assigned U.S. Pat. Nos. 5,170,802 and 5,224,491 and in further U.S. Pat. Nos. 5,387,233, 5,411,546, 5,423,865, and 5,476,498, all incorporated herein by reference. These coronary sinus leads use the shape of the lead body or the distal electrode to lodge against and extend the coronary sinus or great vein wall to hold the distal lead end at the desired site in relation to the left atrium or ventricle.
The '491 patent discloses the use of a balloon expandable or a self expanding, distal, stent-like electrode that is expanded or released and expands in the coronary sinus in order to distribute the electrode surface area over a wide area and to hold the distal lead end in place. A similar stent-like, cardioversion/defibrillation electrode that is used in other cardiac vessels is disclosed in U.S. Pat. No. 5,531,779, incorporated herein by reference. In U.S. Pat. No. 5,221,261, incorporated herein by reference, a number of embodiments of a stent retention mechanism for a catheter that dispenses fluids through a catheter body lumen are disclosed. It is suggested in regard to the first embodiment that the catheter body distal end can bear an electrode that is electrically connected to a conductor extending through the catheter lumen. Upon expansion of the stent retention mechanism, the catheter distal end is located centrally within the vessel lumen and separated from the vessel wall. This provides an inadequate electrode location for cardiac sense electrodes that are best applied directly against cardiac tissue. Moreover, blood flows around the periphery of the tip electrode rather than through a lumen of the distal electrode.